[0001] This invention relates to a process for the preparation of useful thermoplastic resins
in bead form by suspension copolymerization. More particularly, the invention provides
a method for producing styrene/methacrylic acid copolymer beads of uniform size.
BACKGROUND OF THE INVENTION
[0002] Copolymers of styrene and methacrylic acid are thermoplastics known to possess a
number of useful properties, including good dimensional stability and processability.
Articles molded from such copolymers display good resistance to hot water and are
able to withstand steam pasteurization. The heat resistance of the copolymers is much
greater than that of polystyrene. Furthermore, the heat distortion temperature may
be controlled as desired by varying the methacrylic acid content. Styrene/methacrylic
acid copolymers have excellent clarity and high chemical resistance, especially to
oil. Because of their desirable combination of properties, styrene/methacrylic acid
copolymers are ideal for use in appliances, electrical equipment housings, microwave
food containers, automotive parts, medical equipment, packaging, and optical parts.
[0003] Foamed articles may be prepared from styrene/methacrylic acid copolymer by impregnating
beads of the copolymer with a volatile blowing agent and expanding the impregnated
beads at elevated temperature in a mold. The expanded beads fuse together to form
the molded foam article. For this application, the copolymer beads should be essentially
spherical, uniform in size, and sufficiently large so as to avoid the handling problems
encountered with the use of finely powdered resins. If beads smaller or greater than
the optimum size are used, they tend to absorb respectively either too much or too
little blowing agent with respect to the majority of the beads with the result that
the final molded article has a non-uniform cell structure. Very large beads are not
suitable for moldings having intricate surfaces or thin cross-sections. Fine beads
segregate readily during storage to the bottom of a storage container, tend to carry
a greater static charge, and have a relatively short shelf-life because of their
more rapid release of blowing agent during storage. Beads which are spherical and
which have a narrow size distribution will permit closer and more uniform packing
of the beads during foaming, resulting in a higher quality molded article.
[0004] A number of different processes for the preparation of styrene/methacrylic acid copolymers
have been described in the prior art, including bulk or solution polymerization (as
taught in U.S. Pat. Nos. 3,035,033 and 4,275,182, for example).
[0005] One type of process which has been found to be particularly well-suited for styrene/methacrylic
acid copolymerization is suspension polymerization, in which monomers initially suspended
in water as liquid droplets are converted to solid copolymer beads. One of the primary
advantages of a suspension polymerization is that temperature control is relatively
simple due to the ability of the water phase to dissipate the heat of reaction and
the low viscosity of the polymer suspension.
[0006] U.S. Pat. No. 3,839,308 teaches a suspension polymerization process in which the
methacrylic acid is continuously introduced until 50% conversion of the styrene monomer
is achieved. According to the teachings of this patent, copolymers substantially homogeneous
in character and having high heat distortion temperature and tensile strength may
be obtained.
[0007] U.S. Pat. No. 4,631,307 teaches the preparation of rubber-modified styrene/methacrylic
acid copolymers using either emulsion polymerization in combination with a coagulation
step or bulk polymerization followed by suspension polymerization.
[0008] Jpn. Pat. No. 61-252209 teaches a suspension polymerization process in which the
methacrylic acid is added during the initial stage of polymerization. The suspension
preferably contains an emulsion polymerization inhibitor to reduce the formation of
unrecoverable finely powdered copolymer. Higher water/monomer ratios were found to
yield products having superior physical properties.
[0009] Jpn. Pat. No. 60-248708 teaches polymerization of styrene and methacrylic acid using
a free radical initiator. The monomers are partially polymerized in bulk, and then
treated with water and partially saponified polyvinyl acetate to create a suspension
polymerization system.
[0010] U.S. Pat. No. 4,385,156 discloses a suspension polymerization process for producing
coated styrenic polymer beads wherein styrene and methacrylic acid are copolymerized
in the presence of "seed" beads which form the core of the coated beads.
[0011] However, the prior art methods for the suspension polymerization of styrene and methacrylic
acid yield relatively small beads ranging in average size from about 150 to 1100 microns
and having a broad size distribution. Suspension polymerization of styrene and methacrylic
acid is complicated by the water-solubility and polarity of the methacrylic acid.
As a result, methods which work well for controlling bead size and suspension stability
in the polymerization of water-insoluble non-polar monomers such as styrene are not
ordinarily directly suitable for use in styrene/methacrylic acid copolymerizations.
[0012] It is apparent there exists a need for an improved suspension process for the copolymerization
of styrene and methacrylic acid which will produce copolymer beads large and uniform
in size and directly usable in foam applications.
SUMMARY OF THE INVENTION
[0013] This invention provides a process for producing styrene/methacrylic acid copolymer
beads having a narrow size distribution. The process of this invention is particularly
well-suited for the production of copolymer beads sufficiently uniform in size that
at least about 75%, more preferably at least about 85%, of the beads have diameters
falling within a range of about 0.5 mm (for example, from about 0.6 to 1.1 mm).
[0014] In the first step of the process, a biphasic mixture of water, styrene, methacrylic
acid, a water-soluble inorganic salt, and a free radical polymerization initiator
is reacted until from about 5 to 50 percent of the styrene and methacrylic acid are
copolymerized. A water-soluble organic polymer such as polyvinyl alcohol is then added
to the biphasic mixture in an amount effective to form a plurality of discrete liquid
droplets containing copolymer and unreacted monomer suspended in a continuous aqueous
phase. The styrene and methacrylic acid are further reacted until the liquid droplets
are converted into solid beads of styrene/methacrylic acid copolymer. The solid beads
are then separated from the aqueous phase.
[0015] The spherical copolymer beads produced in accordance with the process of this invention
are remarkably uniform in size and can be made sufficiently large in size to enable
direct use in foamed bead applications without an intermediate pelletization step.
[0016] Molded articles obtained by molding the solid beads of copolymer produced by this
process exhibit physical properties, including heat distortion resistance and low
moisture absorbance, at least equivalent to those exhibited by styrene/methacrylic
acid copolymers produced by prior art processes. A further advantage of the process
of this invention is that the composition of the copolymer obtained is very close
to that of the monomer mixture initially charged. Copolymer properties can thus be
precisely controlled while minimizing loss of the the methacrylic acid. The present
process does not require the use of a water-insoluble inorganic suspending agent such
as tricalcium phosphate to stabilize the suspension or to control bead size. Removing
such agents generally requires an additional separate acid wash step during bead recovery.
If not removed, the residual agents may reduce the clarity of articles molded from
the copolymer beads or present toxicity concerns if the final molded product is to
be used in a food service application.
DETAILED DESCRIPTION OF THE INVENTION
[0017] The copolymer produced in accordance with the process of this invention is comprised
of styrene and methacrylic acid. Although the relative proportions of these monomers
are not critical, preferably from about 40 to 99 weight percent of styrene and from
about 1 to 60 weight percent of methacrylic acid are present. In view of the high
water solubility of methacrylic acid, it was surprising to find that copolymers having
exceptionally high levels (>30%) of methacrylic acid could be readily prepared by
the process of this invention. Because of the difficulties encountered in molding
high methacrylic acid content products, however, the copolymer is more preferably
comprised of from about 70 to 92.5 weight percent styrene and from about 7.5 to 30
percent methacrylic acid. The glass transition temperature of the copolymer may be
readily varied as desired by changing the methacrylic acid content. The copolymer
is substantially random in structure; that is, it does not contain any "blocks" or
segments containing only one type of monomer unit. The number average molecular weight
of the copolymer may be from about 10,000 to 1,000,000; most typically, the number
average molecular weight is between about 30,000 and 100,000. The molecular weight
may be controlled, if desired, by adding during copolymerization one or more of the
chain transfer agents well-known in the free radical polymerization art.
[0018] In addition to styrene, other vinyl aromatic monomers may be used in the process
of this invention. These other vinyl aromatic monomers should be capable of being
polymerized by free radical means and are preferably liquid at the polymerization
temperature and substantially insoluble in water. Such compounds include, for example,
ar-methyl styrene, ar-ethyl styrene, ar-tert-butyl styrene, ar-chloro styrene, alpha-methyl
styrene, divinyl benzene, vinyl benzylchloride and vinyl naphthalene, as well as other
alkyl- or halo-substituted styrenes. Mixtures of mono-vinyl aromatic monomers may
also be employed. Minor amounts of other ethylenically unsaturated copolymerizable
monomers may also be employed, including unsaturated nitriles such as acrylonitrile.
[0019] One or more other α,β-unsaturated carboxylic acid moieties may be used in minor amounts
in addition to methacrylic acid in the process of this invention. Acrylic acid and
α,β-unsaturated carboxylic acids having from 3 to 6 carbon atoms which can be polymerized
by free radical means are preferred examples of other suitable α,β-unsaturated carboxylic
acid moieties. Acrylate esters such as methyl methacrylate may also be used. The α,β-unsaturated
carboxylic acid moiety should be liquid and substantially soluble in styrene at the
polymerization temperature.
[0020] In one embodiment of the process of this invention, styrene, methacrylic acid, a
water-soluble inorganic salt, and water are combined in a reactor to form a biphasic
mixture. The reactor should be capable of being stirred, heated, and cooled and may
be of any type generally suitable for use in a suspension polymerization process.
The mixture is stirred and heated at a suitable temperature in the presence of a free
radical polymerization initiator to begin polymerization. Alternatively, the water-soluble
inorganic salt may be added to the biphasic mixture after polymerization has been
initiated.
[0021] The particular water-soluble inorganic salt employed is not critical, although it
is important that the salt does not interfere with the polymerization or adversely
affect the properties of the polymer product. Preferably, the salt is essentially
neutral (that is, not highly acidic or basic) and not readily oxidized or reactive
with free radicals. Suitable water-soluble inorganic salts include, but are not limited
to, alkali metal and alkaline earth halides such as sodium chloride, sodium bromide,
potassium chloride, potassium bromide, magnesium chloride, and calcium chloride, alkali
metal or alkaline earth sulfates such as sodium sulfate, potassium sulfate, and magnesium
sulfate, and alkali metal or alkaline earth nitrates such as sodium nitrate and potassium
nitrate. Mixtures of these or other water-soluble inorganic salts may be used.
[0022] The concentration of water-soluble inorganic salt should be sufficient to provide
a more narrow bead size distribution relative to the bead size distribution obtained
in the absence of the water-soluble inorganic salt. This amount will vary depending
on the water:monomer ratio, the styrene:methacrylic acid ratio, the type and concentration
of the suspending agent, the agitation rate, as well as other reaction parameters,
but is preferably from about 0.5 to 5 percent (most preferably, from about 1 to 2
weight percent) by weight of the aqueous phase. Higher concentrations are unnecessary
and can result in precipitation of the water-soluble organic polymer. A suitable salt
concentration for a particular set of reaction conditions can be easily determined
by adding incremental amounts of salt to a partially polymerized reaction mixture
until the desired degree of dispersion of the organic phase into discrete liquid droplets
is observed.
[0023] The beneficial results obtained by the use of the water-soluble inorganic salt were
completely unexpected, as suspension polymerizations are generally performed using
water which has been carefully purified to remove dissolved salts. It is well known
in other suspension polymerization processes that the presence of ionic substances
can result in inhibition of polymerization or contamination of the polymer product.
[0024] The free radical polymerization initiator may be any of the organic-soluble initiators
well known in the suspension polymerization art such as organic peroxides, peresters,
perketals or percarbonates. A mixture of free radical polymerization initiators may
be advantageously used; mixtures of a low temperature and a high temperature initiator
have been found to be particularly effective. If a mixture of initiators is employed,
they may be introduced at different times during the copolymerization. Likewise, if
only one initiator is used, it may be added in portions while the copolymerization
is proceeding. Examples of low temperature initiators include organic peroxides such
as benzoyl peroxide, caproyl peroxide, lauroyl peroxide, t-butyl peroctoate, cyclohexanone
peroxide, and decanoyl peroxide, as well as other initiators such as azo-bis-isobutyronitrile
(AIBN). Illustrative high temperature initiators include, t-butyl peracetate, t-butylperbenzoate,
or t-butyl peroxy isopropyl carbonate. The amount of low temperature initiator preferably
varies from about 0.03 to 1.0%, most preferably from about 0.08 to 0.25%, by weight
based on total weight of the monomers. The high temperature initiator preferably is
employed in amounts varying from about 0.01 to 0.25%, most preferably from about 0.05
to 0.15%, by weight based on total weight of the monomers. In any case, the total
amount of initiator should be sufficient to achieve substantial (preferably, over
95%) conversion of the monomers under the reaction conditions used.
[0025] The weight ratio of water to total monomer used in the process of the invention is
not critical. In general, ratios ranging from about 1:1 to 4:1 are suitable.
[0026] The biphasic mixture comprising water, styrene, methacrylic acid, and the free radical
initiator is heated with stirring, preferably at a temperature of between about 60°C
and 100°C, to initiate polymerization of the monomers. It may be desirable to initiate
polymerization with only a portion of one monomer present and then add the remainder
of that monomer continuously to the reaction mixture. This and other variations in
combining the monomers are considered to be within the scope of this invention. During
the first phase of the polymerization, the mixture will consist of an organic phase
and an aqueous phase. The mixture will generally be milky-white in appearance if sufficient
agitation is applied. Under these conditions, the organic phase (containing the majority
of the styrene and methacrylic acid copolymer) will be finely dispersed in the aqueous
phase. If the rate of agitation is low, however, the monomers will comprise a substantially
continuous organic phase. If agitation of the stirred biphasic mixture is stopped,
the finely dispersed organic phase readily forms a substantially continuous organic
phase. The degree of agitation during the first phase of the polymerization is not
critical, as good results can be obtained independent of the agitation rate. As polymerization
proceeds, the copolymer formed will be largely dissolved in the organic phase.
[0027] Monomer conversion and viscosity are monitored until from about 5 to 50% of each
of the monomers has reacted and the organic phase has begun to thicken noticeably
due to copolymer formation. The viscosity of the organic phase at this point is preferably
not greater than about 200 poise at 25°C; more preferably, the viscosity does not
exceed about 50 poise. A water-soluble organic polymer in an amount effective to convert
the organic phase into a plurality of discrete liquid droplets is then introduced
into the biphasic mixture. The discrete liquid droplets are comprised of the copolymer
and unreacted monomers and are substantially the same size as the desired size of
final copolymer beads. From the time at which the water-soluble organic polymer is
added to the completion of polymerization, sufficient agitation is supplied to prevent
agglomeration of the discrete droplets without breaking up the droplets into an overly
fine emulsion. The precise agitation rate is not critical and can vary widely due
to the inherent stability of the suspension.
[0028] The water-soluble organic polymer is most preferably added when from about 10 to
20% monomer conversion has occurred. The average bead size can generally be controlled
as desired by adjusting the time at which the water-soluble organic polymer is introduced
into the biphasic mixture. For example, larger beads will typically be obtained by
adding the water-soluble organic polymer when a higher degree of monomer conversion
has been achieved.
[0029] Any water-soluble organic polymer suitable for forming the discrete droplets may
be used. Preferred water-soluble organic polymers include polyvinyl alcohol, partially
saponified poly(vinyl acetate), water-soluble cellulose derivatives such as methyl
cellulose, hydroxy ethyl cellulose, and carboxyl methyl cellulose, poly(α,β-unsaturated
carboxylic acids) such as poly(acrylic acid), and poly(vinyl pyrrolidinone). Of these,
polyvinyl alcohol is the most preferred since relatively low concentrations are generally
quite effective in converting the organic phase into liquid droplets. Polyvinyl alcohol
having a molecular weight of from about 75,000 to 110,000 is most favored for use
in the process of this invention. An example of a suitable polyvinyl alcohol is VINOL®
540, a product of Air Products and Chemicals. The amount of water-soluble polymer
required to achieve the desired formation of liquid droplets will vary depending on
the particular water-soluble organic polymer used, styrene/methacrylic acid ratio,
temperature, agitation rate, and water/monomer ratio among other factors. From about
0.005 to 1.0% (more preferably, from about 0.02 to 0.1%) polyvinyl alcohol based on
the total weight of water and monomer will usually be effective, for example.
[0030] In general, however, the water-soluble organic polymers useful in the process of
this invention are most suitably used in concentrations from about 0.005 to 10.0%
by weight of the total biphasic mixture.
[0031] It is important to add the water-soluble organic polymer to the biphasic mixture
after polymerization has been initiated. For reasons that are not well understood, attempts
to obtain spherical copolymer beads of uniform size were unsuccessful when this sequence
of steps was reversed.
[0032] After addition of the water-soluble organic polymer, copolymerization of the remaining
unreacted monomers is continued until substantially all (preferably, over 95%) of
the monomer has reacted and the liquid droplets have been converted to solid beads
of copolymer. Completion of polymerization may be carried out at temperatures comparable
to or slightly higher than those used during the initial part of the polymerization.
More preferably, though, the reaction temperature is increased to over 100°C in order
that all of the residual monomer is reacted more quickly. It is preferred that this
final temperature not exceed the glass transition temperature of the copolymer, however,
so that the beads do not soften and agglomerate.
[0033] The slurry of solid copolymer beads in water obtained by the process of this invention
may then be treated to separate the beads from the aqueous phase. Methods such as
filtration, decantation, or centrifugation are suitable for this purpose. The separated
beads may be washed with water or other suitable solvent to remove residual impurities,
particularly any water-soluble polymer or inorganic salt which may be on the surface
of the polymer beads. The separated beads can then be dried by any suitable method
to remove water and other volatile residues. Sieving may be employed if desired to
separate any small amount of fine powder or overly large beads from the copolymer
beads of the desired size. This is generally not necessary, however, due to the exceptionally
narrow size distribution afforded by the process of this invention.
[0034] If desired, the fine powder obtained after sieving the bead copolymer product can
be readily recycled in subsequent copolymerizations. The fine powder is apparently
incorporated into larger sized beads since the relative proportion of fine powder
produced in the subsequent run is not larger than is observed in the absence of the
fine powder. Thus, accumulation of the fine powder by-product, which may be too small
to be used directly in a foamed bead application, can be easily avoided.
[0035] In one embodiment of this invention, one or more rubbery polymers are added to the
biphasic mixture to produce a rubber-modified copolymer. The rubber-modifier greatly
improves the impact strength of the copolymer and reduces the brittleness of the product.
The rubbery polymer preferably has a glass transition temperature below 0°C (more
preferably, below -30°C) and contains at least one ethylenically unsaturated functional
group which provides a site for grafting onto the styrene/acrylic acid monomer copolymer.
It is desirable to combine the rubbery polymer with the comonomers in the first step
of the process before polymerization is initiated.
[0036] Exemplary rubber polymers for use in preparing impact-modified copolymer beads include
polybutadiene, polyisoprene, styrene-butadiene copolymers (block or random), butadiene-acrylonitrile
copolymer, polychloroprene, polyisobutylene, ethylene-propylene copolymer, acrylonitrile-butadiene-styrene
terpolymers, ethylene-propylene-diene (EPDM) terpolymers, butadiene-acrylate copolymers,
polypentenamers, alkyl acrylate polymers and copolymers, ethylene-vinyl acetate copolymers,
ethylene-alkyl acrylate copolymers, and the like. Other similar impact modifiers known
to those skilled in the art are also suitable. The use of polybutadiene or styrene-butadiene
block or random copolymers is generally preferred, particularly polybutadiene having
a high (>35%) cis-1,4-polybutadiene content.
[0037] The amount of the rubbery polymer which may be used is preferably from about 1 to
70 weight percent, most preferably about 5 to 40 weight percent, of the total weight
of copolymer.
[0038] In another embodiment of this invention, a pigment or dye is added to the biphasic
mixture to produce pre-colored copolymer beads. The copolymer beads thus obtained
can be used directly in the preparation of colored molded articles; no separate dyeing
or melt-blending step is required.
[0039] If desired, a lubricant may be readily incorporated into the copolymer beads by adding
a suitable lubricant to the biphasic mixture before copolymerization is completed.
A separate melt-blending step thus is not needed. In the process of this invention,
the lubricant is most preferably dissolved in the monomers prior to initiation.
[0040] The lubricant-containing beads have a higher bulk density and have improved melt
flow properties and lower color when extruded or otherwise melt-processed. Without
wishing to be bound by theory, it is believed that the lubricant reduces the affinity
of the copolymer for the water-soluble organic polymer. As a result, less of the water-soluble
organic polymer remains in the beads when a lubricant is used. Since the water-soluble
organic polymers usable in the process of this invention tend to be thermally unstable,
color development during extrusion is minimized if a lubricant is present during bead
formation.
[0041] Any suitable lubricant may be employed, including for example long chain (C₁₄-C₃₂)
alkyl alcohols, esters, diesters, or amides of long chain (C₁₄-C₃₂) alkyl alcohols,
long chain (C₁₄-C₃₂) aliphatic carboxylic acids and metallic soaps thereof, polysiloxanes,
hydrocarbon waxes, as well as naturally derived waxes such as beeswax, candelilla,
carnauba, Japan wax, ouricury wax, Douglas-Fir Bark wax, rice-bran wax, jojoba wax,
castor wax, bayberry wax, Montan wax, and peat wax. The use of one or more hydrocarbon
waxes as a lubricant is preferred. Illustrative hydrocarbon waxes include liquid branched
chain paraffins, solid straight chain paraffins, high density polyethylene waxes (preferably
having molecular weights of about 1000 to 9000), low density polyethylene waxes (preferably
having modecular weights of about 1500 to 2500), polypropylene waxes, Fischer-Tropsch
waxes, mineral oil, petroleum jelly, chemically-modified hydrocarbon waxes, microcrystalline
waxes, and semi-crystalline waxes.
[0042] The lubricant is preferably soluble in the styrene/methacrylic acid monomer mixture
and should not interfere with the suspension stability. The amount of lubricant is
not critical, but concentrations in the copolymer product of from about 0.1 to 5.0
weight percent are generally suitable. High levels of lubricant may adversely affect
the heat resistance of molded articles.
[0043] The solid beads of copolymer produced in accordance with the process of this invention
can be molded into useful articles by any suitable molding method, including injection
molding, blow molding, and extrusion molding. The beads may be molded directly or
pelletized into larger beads before molding. Polymer blends and alloys may be obtained
by blending the copolymer beads with other thermoplastic or elastomer resins. It may
be advantageous to incorporate additives such as lubricants, anti-static agents, colorants,
fillers, plasticizers, reinforcing fillers, anti-oxidants, and light stabilizers
into the copolymer beads.
[0044] In one embodiment of this invention, the copolymer beads are impregnated with one
or more volatile blowing agents such as pentane or other low-boiling hydrocarbon,
methylene chloride, carbon dioxide, or a fluorocarbon. The impregnation may be accomplished
by any of the methods generally suitable for impregnating thermoplastic beads. For
example, the copolymer beads may be suspended in an aqueous medium together with the
blowing agent and impregnated at elevated temperature and pressure. The impregnation
may be carried out during or immediately following the suspension copolymerization.
The expandable impregnated beads, once recovered from the aqueous medium, are then
shaped into molded articles by heating. The beads expand and fuse together to form
the molded article. Preferably, the impregnated beads are pre-expanded before the
final molding step. Such methods are described in Ingram et al "Polystyrene and Related
Thermoplastic Foams"
Plastic Foams Marcel Dekker (1973), Part II, Chapter 10, pp. 531-581, InPram "Expandable Polystyrene
Processes"
Addition and Condensation Polymerization Processes American Chemical Society (1969), Chapter 33, pp. 531-535, and U.S. Pat. Nos. 2,983,692;
3,072,581; 3,304,274; 3,126,354; 2,950,261; 2,893,963; 3,085,073; 3,088,925; 2,744,291;
2,787,809; 3,324,052; 3,192,169; 3,265,643; and 4,547,527. The teachings of these
references are incorporated herein in their entirety.
[0045] Without further elaboration, it is believed that one skilled in the art can, using
the preceding description, utilize the present invention to its fullest extent. The
following examples, therefore, are to be considered as merely illustrative and not
limitative of the claims or remainder of the disclosure in any way whatsoever.
EXAMPLE 1
[0046] This example illustrates the preparation of uniformly sized copolymer beads having
a 85:15 styrene/methacrylic acid weight ratio by suspension polymerization in accordance
with the process of this invention.
[0047] A 4 liter resin reactor equipped with a mechanical stirrer, a thermometer, and a
condenser was charged with the following:
|
Wt., g |
Deionized Water |
2100 |
Sodium Sulfate |
16 |
Styrene |
595 |
Methacrylic Acid |
105 |
Benzoyl Peroxide |
2.1 |
[0048] The mixture was heated at 90°C with agitation for 30 minutes until two clearly separated
phases were observed. An aqueous solution of polyvinyl alcohol (5%, 30 g) was then
added, causing the organic phase to disperse into discrete liquid droplets. Polymerization
was continued for another 6 hours at the same temperature. The mixture was cooled
and the product collected on a 200 mesh sieve. After washing with water and drying,
the copolymer beads obtained weighed 680g (97% yield), were almost perfectly spherical
in shape, and had a glass transition temperature (Tg) of 140°C (measured by DSC).
The bead size distribution determined by sieving was exceptionally narrow:
>1.1 mm |
1% |
0.6 - 1.1 mm |
97% |
<0.6 mm |
2% |
EXAMPLE 2
[0049] This example demonstrates that the process of this invention can be performed at
a relatively low water:monomer ratio (in this example, 2:1) while maintaining a stable
low viscosity suspension and uniform bead sized In this example, the water-soluble
inorganic salt was added after polymerization had been initiated.
[0050] The reactor used in Example 1 was charged with the following materials:
|
Wt., g |
Deionized Water |
1800 |
Styrene |
765 |
Methacrylic Acid |
135 |
Benzoyl Peroxide |
2.71 |
[0051] The mixture was agitated at 360 rpm using a stirrer tip speed of about 4.7 ft/sec.
while heating to 90°C. An aqueous solution of sodium sulfate (18g in 100 mL water)
was added 70 minutes after the temperature of the mixture reached 80°C. At 85 minutes,
an aqueous polyvinyl alcohol solution (5%; 40g) was added to disperse the organic
phase into discrete liquid droplets.
[0052] After 170 minutes, a mild exotherm to 92°C lasting about 40 minutes was observed.
Polymerization was continued at 90°C for a total of 5 hours. The solid beads obtained
were water washed and dried to give 870g (97% yield) of copolymer having a Tg of 139°C.
Analysis indicated that nearly 90% of the beads were in the desired size range:
>1.1 mm |
0.1% |
0.6 - 1.1 mm |
88.9% |
<0.6 mm |
11.0% |
EXAMPLE 3
[0053] This example shows that the copolymer obtained in the form of undesirably fine powder
can be recycled to give larger beads. This procedure thus provides a method of preventing
the unwanted accumulation of fine powder.
[0054] A resin reactor was charged with the same amounts of reaction components as in Example
2. Sixty minutes after the reaction temperature had reached 85°C, an aqueous solution
of sodium sulfate (18g in 100 mL water) was added. After 70 minutes, the fine powder
(<0.6 mm) from Example 2 was added to the polymerization mixture. Polyvinyl alcohol
(40 g of a 5% aqueous solution) was added at 80 minutes. The polymerization and product
isolation were completed using the procedures described in Example 2. A 96% yield
of copolymer was obtained. The bead size distribution was comparable to that observed
in Example 2, indicating that no build-up of fines was occurring:
>1.1 mm |
0.3% |
0.6 - 1.1 mm |
89.0% |
<0.6 mm |
10.7% |
EXAMPLE 4
[0055] This example demonstrates that uniformly sized copolymer beads containing a high
level (20 weight percent) of methacrylic acid can be prepared using the process of
this invention.
[0056] The 4 liter resin reactor of Example 1 was charged with:
|
Wt., g |
Deionized Water |
1800 |
Styrene |
720 |
Methacrylic Acid |
180 |
Benzoyl Peroxide |
2.71 |
[0057] After heating 60 minutes at 85-90°C, sodium sulfate (18g in 100 mL water) was added.
Polyvinyl alcohol (40 g of a 5% aqueous solution) was added after 75 minutes. The
copolymerization was continued for a total of 6 hours and the product isolated using
the previously described procedure. A 98% yield of copolymer beads (Tg = 149°C) was
obtained. As in previous examples, the bead size distribution was exceptionally narrow:
>1.1 mm |
1.3% |
0.6 - 1.1 mm |
88.0% |
<0.6 mm |
10.8% |
EXAMPLE 5
[0058] The incorporation of a dye into a styrene/methacrylic acid copolymer during suspension
polymerization while maintaining a narrow bead size distribution is shown by this
example.
[0059] The reactor used in previous examples was charged with the following:
|
Wt., g |
Deionized Water |
1800 |
Styrene |
720 |
Methacrylic Acid |
180 |
Benzoyl Peroxide |
2.71 |
Perox Blue 2 R¹ |
5 mg |
¹ A blue dye produced by Morton Chemical |
[0060] The copolymerization was carried out in exactly the same manner as in Example 4,
including the addition of sodium sulfate prior to the polyvinyl alcohol addition.
The copolymer beads produced possessed an aesthetically pleasing light blue tint and
were quite uniform in size:
>1.1 mm |
0.9% |
0.6 -1.1 mm |
88.6% |
<0.6 mm |
10.5% |
EXAMPLE 6
[0061] The preparation of an impact-modified styrene/methacrylic acid copolymer in bead
form by the process of this invention is demonstrated by this example.
[0062] A 4 liter resin kettle equipped as described in Example 1 was charged with the following
components:
|
Wt., g |
Deionized Water |
2000 |
Sodium Sulfate |
20 |
Styrene |
727 |
Methacrylic Acid |
128 |
Butadiene/Styrene Rubber¹ |
150 |
Benzoyl Peroxide |
1.0 |
Dilauroyl Peroxide |
3.0 |
[0063] The monomers and butadiene/styrene rubber are mixed before adding the other components
in order to dissolve the rubber. The copolymerization was conducted using the procedure
described in Example 1. A high yield (93%) of spherical uniformly sized copolymer
beads was obtained. The impact strength of a molded article prepared using these beads
exhibited a Notched Izod of 2.51 ft.lbs/in, compared to 0.3 for an article made by
molding the copolymer beads of Example 1.
EXAMPLES 7-9
[0064] To demonstrate the use of other water-soluble organic polymers in the process of
this invention, the procedure of Example 1 is repeated using 37.5g hydroxyethyl cellulose
(Example 7), 4.5g 75% saponified poly(vinyl acetate) (Example 8), or 28.0g polyacrylic
acid (Example 9) in place of the polyvinyl alcohol. In each case, a high yield of
spherical uniformly sized copolymer beads should be obtained.
EXAMPLES 10-12
[0065] To demonstrate the use of other water-soluble inorganic salts in the process of this
invention, the procedure of Example 1 is repeated using 7.0g sodium chloride (Example
10), 12.0g potassium nitrate (Example 11), or 13.0 g calcium chloride (Example 12)
in place of the sodium sulfate. In each case, a high yield of spherical uniformly
sized copolymer beads should be obtained.
EXAMPLE 13
[0066] The impregnation of the styrene/methacrylic acid copolymer beads prepared by the
process of this invention with pentane is illustrated by this example.
[0067] A 12 oz. crown capped bottle is charged with the following:
|
Wt., g |
Water |
150 |
Copolymer Beads (Example 1) |
100 |
Tricalcium Phosphate |
1.33 |
Sodium Dodecylbenzene Sulfate |
0.04 |
Polyoxyethylene (20) Sorbitan Monolaurate |
0.10 |
n-Pentane |
11.5 |
Sodium Chloride |
4.5 |
[0068] The bottle is sealed and then rotated end-over-end for 3 hours at 90°C and 8 hours
at 110°C in an oil bath. The bottle is cooled to room temperature, opened, and the
contents acidified with hydrochloric acid. The impregnated beads containing n-pentane
are separated from the aqueous medium by filtration, washed with water, and air dried.
The impregnated copolymer beads are pre-expanded in a Kohler General Drispander (a
reduced pressure double-layer vessel) using a steam pressure of 50 psi to give pre-expanded
beads having a volume 10-40 times the original volume. The pre-expanded beads are
then formed into a molded article by heating the beads in a closed mold at a temperature
sufficient to accomplish further expansion and fusion of the beads.
EXAMPLES 14-20
[0069] The preparation of styrene/methacrylic acid copolymer beads containing a lubricant
is illustrated by these examples. A polyethylene wax or paraffin wax in the amount
shown in Table I was dissolved in the monomer mixture before carrying out the copolymerization
procedure described in Example 1. The lubricant-containing copolymer beads were spherical,
remarkably uniform in size, and had higher bulk densities than the beads of Example
1. In addition, the melt flow properties as measured by melt index were significantly
improved and the extruded products were much lower in color than when no lubricant
was used.
COMPARATIVE EXAMPLE 21
[0070] This example shows that the size distribution of beads produced by a suspension polymerization
using tricalcium phosphate and polyvinyl alcohol, but no water-soluble inorganic salt,
is considerably broader than the distribution obtained using the process of this invention.
In this example, the polyvinyl alcohol was added before polymerization was initiated
in contrast to the present invention process.
[0071] The 4 liter resin reactor of Example 1 was charged with the following:
|
Wt., g |
Deionized Water |
1800 |
Styrene |
850 |
Methacrylic Acid |
150 |
Benzoyl Peroxide |
3.0 |
Tricalcium Phosphate |
10 |
Polyvinyl Alcohol |
1.5 |
[0072] The mixture was heated 6 hours at 90°C before collecting the copolymer product in
the same manner as described in the previous examples. A 95% yield of copolymer beads
having a Tg of 139°C was obtained. However, the bead size distribution was very broad;
a large proportion of fine powder (<0.6 mm) was present.
>1.1 mm |
1.1% |
0.6 -1.1 mm |
25.4% |
0.425 - 0.6 mm |
36.6% |
0.250 - 0.425 mm |
30.9% |
0.150 - 0.250 mm |
6.0% |
TABLE I
Effect of Waxes on the Properties of Styrene/Methacrylic Acid Copolymers |
EXAMPLE NO. |
LUBRICANT@ |
WT.% |
BULK DENSITY g/100 mL |
MELT INDEX# g/10 min. |
Tg, °C |
APPEARANCE# |
1 |
none |
0 |
45.0 |
1.2 |
139 |
light yellow |
14 |
Bareco® 655 |
0.5 |
46.5 |
2.0 |
138 |
clear |
15 |
Bareco® 655 |
1.0 |
52.7 |
2.7 |
136 |
clear |
16 |
Bareco® 1000 |
1.0 |
47.0 |
1.9 |
140 |
white |
17 |
Bareco® 1000 |
2.5 |
47.0 |
2.3 |
140 |
white |
18 |
Paraffin Wax |
1.0 |
47.0 |
2.7 |
130 |
clear |
19 |
Paraffin Wax |
2.5 |
57.0 |
5.6 |
127 |
clear |
20 |
Paraffin Wax |
5.0 |
50.0 |
8.3 |
124 |
clear |
@ The Bareco® waxes are polyethylene waxes available from Petrolite Inc. The paraffin
wax is the microcrystalline petroleum wax sold by Gulf Oil Company for use in candle
making. |
# The appearance of the strand from a melt index machine was examined. The ASTM D-1238
Condition L was followed. |
1. A process for producing styrene/methacrylic acid copolymer beads having a narrow
size distribution, the process comprising the steps of:
(a) reacting a biphasic mixture comprising water, styrene, methacrylic acid, a water-soluble
inorganic salt, and a free radical polymerization initiator until from about 5 to
50 percent of the styrene and methacrylic acid are copolymerized;
(b) adding a water-soluble organic polymer to the biphasic mixture in an amount effective
to form a plurality of discrete liquid droplets containing styrene/methacrylic acid
copolymer and unreacted styrene and methacrylic acid suspended in a substantially
continuous aqueous phase;
(c) further reacting the styrene and methacrylic acid until the liquid droplets are
converted into solid beads of styrene/methacrylic acid copolymer; and
(d) separating the solid beads from the aqueous phase;
wherein the amount of water-soluble inorganic salt is sufficient to provide a more
narrow bead size distribution relative to the bead size distribution obtained in the
absence of the water soluble inorganic salt.
2. The process of claim 1 wherein the weight ratio of styrene to methacrylic acid
is from about 99:1 to 40:60.
3. The process of claim 1 wherein the weight ratio of styrene to methacrylic acid
is from about 92.5:7.5 to 70:30.
4. The process of any one of claims 1 to 3 wherein the weight ratio of water to the
total amount of styrene and methacrylic acid is from about 1:1 to 4:1.
5. The process of any one of claims 1 to 4 wherein the water-soluble inorganic salt
is selected from alkali metal halides, alkaline earth halides, alkali metal sulfates,
alkaline earth sulfates, alkali metal nitrates, alkaline earth nitrates, and mixtures
thereof.
6. The process of claim 5 wherein the water-soluble inorganic salt is selected from
sodium chloride and sodium sulfate.
7. The process of any one of claims 1 to 6 wherein the weight ratio of water-soluble
inorganic salt to water is from about 0.5:99.5 to 5:95.
8. The process of any one of claims 1 to 7 wherein the water-soluble inorganic salt
is added to the biphasic mixture after polymerization has been initiated and before
step (b).
9. The process of any one of claims 1 to 8 wherein reaction step (a) is carried out
until from about 10 to 20 percent of the styrene and methacrylic acid are copolymerized.
10. The process of any one of claims 1 to 9 wherein the water-soluble organic polymer
is selected from partially saponified poly(vinyl acetate). water-soluble cellulose
derivatives, poly(α,β-unsaturated carboxylic acids), poly(vinyl pyrrolidinone), and
mixtures thereof.
11. The process of any one of claims 1 to 9 wherein the water-soluble organic polymer
is poly(vinyl alcohol).
12. The process of any one of claims 1 to 11 wherein the amount of water-soluble organic
polymer is from about 0.005 to 1.0 percent of the total weight of the biphasic mixture.
13. The process of any one of claims 1 to 12 wherein at least one of a dye, a rubbery
polymer and a lubricant is additionally present in the biphasic mixture.
14. The process of claim 13 wherein the rubbery polymer has at least one ethylenically
unsaturated functional group capable of grafting onto the styrene/methacrylic acid
copolymer.
15. The process of claim 13 wherein the lubricant is a hydrocarbon wax.
16. The process of any one of claims 1 to 15 comprising the additional step after
step (d) of washing and drying the copolymer beads.
17. The process of any one of claims 1 to 16 comprising the additional step after
step (c) of impregnating the copolymer beads with a blowing agent.
18. A process for making a molded article comprising molding styrene/methacrylic acid
copolymer beads produced in accordance with the process of any one of claims 1 to
17.